Document reading devices typically include a document table on which multiple original documents can be stacked, an automatic document feeder (ADF) to transport the multiple original documents one by one from the document table to an image reading position, and an image reading unit to read image data of the original document. ADFs include a pickup roller that applies a transport force to the top sheet of a bundle of original documents stacked on the document table toward a separation unit. The separation unit includes, for example, a feed roller and a separator pressed against the feed roller, forming a nip (separation nip) therebetween. The separator separates the top sheet from the rest of original documents to feed each of the multiple original documents one by one to the image reading position.
In addition, for sequential sheet conveyance, ADFs include a trailing-edge detector disposed downstream from the separation nip in the direction in which the original document is transported (hereinafter “sheet conveyance direction”). The trailing-edge detector detects the trailing end of the original document that has passed through the separation nip (hereinafter “the preceding sheet”), which triggers feeding of a subsequent sheet from the multiple original document. The trailing-edge detector may be a reflection-type or transmission-type photosensor that directs light onto a surface of the original document to detect its presence, thereby determining whether the trailing end of the original document has passed by a predetermined detection position.
There is increasing demand for improving productivity in sequential sheet conveyance and streamlining the operation. Accordingly, various approaches are tried to reduce intervals between multiple original documents transported sequentially by ADFs.
For example, JP-2005-324872-A proposes increasing the velocity at which originals are transported (hereinafter “conveyance velocity of originals”) through the separation nip from the conveyance velocity of original documents at the reading position in the above-described configuration, in which feeding of the subsequent sheet is triggered when the trailing end of the preceding sheet passes through a predetermined position downstream from the separation nip in the sheet conveyance direction. In this approach, while the preceding sheet is passing by the reading position, the subsequent sheet is transported through the separation nip at a velocity faster than the velocity at which the preceding sheet is transported by the reading position. Consequently, the interval between the sheets in sequential sheet conveyance can be reduced from when feeding of the subsequent sheet is started, thus enhancing productivity.
However, as the conveyance velocity at the reading position increases it becomes difficult to provide a significant difference between the velocity at which the separating unit transports original documents and the velocity at which the original document passed through the reading position for reducing intervals between sheets in sequential sheet conveyance. Therefore, it is preferred to reduce the interval between the preceding sheet and the subsequent sheet at the start of feeding the subsequent sheet. Although this objective may be attained by disposing the trailing-edge detector closer to the separation nip, it is possible that the leading end of the subsequent sheet transported together with the preceding sheet can project downward in the sheet conveyance direction beyond the separation nip, in which case the leading end of the subsequent sheet may face the trailing-edge detector and thus inhibit the trailing-edge detector from detecting the trailing end of the preceding sheet. Accordingly, it is difficult to dispose the trailing-edge detector sufficiently close to the separation unit to reduce intervals between sheets significantly.